Polyaniline/bentonite was used in this study to investigate its adsorption behaviors of reactive green 19. Influencing factors including contact time, pH, temperature and inorganic salts were investigated. The adsorption kinetic data fitted the pseudo-second-order model better than the pseudo-first-order and Elovich models, and the Langmuir model was better than the Freundlich model to describe the adsorption process. Thermodynamic studies indicated that the adsorption of reactive green 19 by polyaniline/bentonite was an endothermic and spontaneous process. Experimental data indicated that both NaCl and Na2SO4 could improve the reactive green 19 adsorption as a result of aggregation of the dye molecules in solution induced by the inorganic salts, and an increase of pH value from 5 to 9 caused a slight decrease in reactive green 19 adsorption.

INTRODUCTION

China produced approximately two-thirds of the total global textile fibers in 2012, ranking in the top three largest textile manufacturers worldwide (Peng et al. 2015). Hence, large quantities of dyes have to be generated to meet the needs of the textile production. At present, China is the largest producer of dyes in the world. Data from the China Dyestuff Industry Association (CDIA) show that the annual production of commercial dyes was 8.95 × 105 tons in 2013, accounting for about 65% of the world yield. The huge dye applications have attracted increasing public concern because about 30% of the used dyestuffs are lost during dyeing processes, to generate large volumes of dye-bearing effluents with high concentrations of dyes. The total discharge of dyeing wastewater was 1.96 × 109 m3 in 2013 in China (Wang et al. 2015a, 2015b). The organics, bleaches and salts in dyeing effluents can not only cause deterioration of the quality of receiving water bodies, but can also destroy the aquatic ecosystems and even cause serious health issues since some dyes or their degradation intermediates are cancerogenic, teratogenic and mutagenic (Gong et al. 2009; Tang et al. 2014). However, the chemical stability resulting from their mainly aromatic structures makes the dyes resistant to traditional biodegradation (Ewa & Gra 2007; Wang & Wang 2008). Therefore, physicochemical techniques including adsorption, ozonation, nanofiltration, coagulation and photocatalytic oxidation etc. have been developed to treat dye-containing effluents (Mahmoodi 2013; Ong et al. 2013; Zhang et al. 2013; Liang et al. 2014; Rosa et al. 2014; Hua et al. 2015; Kim et al. 2015; Rosa et al. 2015; Wang et al. 2015a, 2015b; Zhu et al. 2015). Among these methods, adsorption has been recognized as the most attractive process for dye removal from aqueous solution due to its easy operation, high performance and low cost. The effectiveness and efficiency of the adsorbent is the decisive factor for the adsorption technique. Many adsorbents including both synthetic and natural materials, such as activated carbon, chitosan, agricultural wastes, lignite, sepiolite and so on, have been tested to remove dyes from their aqueous solutions (Dogan et al. 2009; Dotto et al. 2014; Gürses et al. 2014; Zarezadeh-Mehrizi & Badiei 2014; Aboua et al. 2015; Duman et al. 2015).

Bentonite is an attractive adsorbent due to its advantages such as large specific surface area, good cation exchange capacity and excellent chemical and physical stability (Zaghouane-Boudiaf et al. 2014), and it has been used successfully in water treatment as an adsorbent to remove many pollutants such as heavy metals, organic compounds and viruses (Anirudhan & Suchithra 2010; Toor & Jin 2012; Jovic-Jovicic et al. 2013; Zha et al. 2013; Hao et al. 2014; Okabe et al. 2014; Bellou et al. 2015). In China, use of bentonite as an adsorbent for water purification is of particular convenience as a result of its largest proven reserve of 5,087 million tons and annual production of more than 3.5 million tons in the world. However, natural bentonite has poor capacity for adsorption of anionic dyes because its negatively charged surface, resulting from the isomorphous substitution of Al3+ for Si4+ in its tetrahedral layer, and Mg2+ for Al3+ in the octahedral layer, repulses the dye molecule with the same charge in solution (Özer Gök et al. 2010; Toor & Jin 2012). Hence, modification of bentonite is very important to improve its adsorption of anionic dyes.

In this work, polyaniline/bentonite composite was synthesized chemically to improve the adsorption capacity of raw bentonite. An anionic dye, reactive green 19 (RG-19), was selected as a target pollutant to test the adsorption characteristics of the polyaniline/bentonite. The effects of various factors including pH, reaction time, temperature and inorganic salts etc. were investigated.

MATERIALS AND METHODS

Preparation of adsorbent

The adsorbent used in this study was synthesized in a manner developed from the modification of the methods described in the published documents Linares & Torres (2005), Motawie et al. (2014) and Yapara et al. (2005). The specific steps are detailed as follows.

Purified bentonite

The mixture of raw bentonite powder purchased from Inner Mongolia, China, and deionized water was stirred at 4,000 rpm for 0.5 h and then treated by ultrasonic wave for another 0.5 h. The upper layer of the mixture after settlement for 10 min was moved into a beaker and settled for another 24 h. The suspension in the beaker was collected, centrifuged and dried to get the purified bentonite (Puri-Bent).

Sodium bentonite

The sodium bentonite was prepared by dispersing the purified bentonite into 4 wt% Na2CO3 solution, followed by being mixed at 80°C for 2 h. The mixture was then centrifuged and the deposit was washed with deionized water until free of Na2CO3, dried at 105°C, and ground.

Organo-bentonite

The sodium bentonite was dispersed in 20 wt% hexadecyl trimethyl ammonium bromide solution, treated with ultrasonic wave for 30 min, and mixed at 70°C for 2 h in sequence. The mixture was centrifuged and the deposit was washed with deionized water until free of bromide, dried at 80°C for 12 h, and ground.

Polyaniline/bentonite

The organo-bentonite was mixed with aniline at the ratio of 1:4 (W:W) for 72 h. The mixture was centrifuged and washed with deionized water until free of aniline and then dried at 80°C for 12 h to get the aniline/bentonite. A total of 5 g of aniline/bentonite was mixed with 20 ml of deionized water containing 50 mg of cyclohexanone peroxide and 2.5 mg of cobalt iso-octoate. The mixture was stirred in an ice bath for 0.5 h and then washed with deionized water, dried and ground to get the polyaniline/bentonite (Pani-Bent).

Characterization of the adsorbent

X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the adsorbent (Bober et al. 2010). XRD patterns of the Puri-Bent and Pani-Bent were acquired with an X-ray diffractometer (X'TRA, ARL Co. Ltd, Switzerland) over the scanning range of 2θ = 2° − 80° to study the changes in their structural properties. Bragg's law n λ = 2d sinθ was used to calculate the d001 of the two samples. FTIR spectra were obtained by using a FTIR Spectrum (Nicolet 5700, Thermo Nicolet Ltd, USA) to observe the surface functional groups of the two samples.

Preparation of artificial wastewater

RG-19 (chemical formula: C40H23Cl2N15Na6O19S6, molecular weight: 1418.93 g mol−1) purchased via the internet was used without further purification. Its structure is shown in Figure 1. The RG-19 solution used in this study was prepared by adding a certain amount of RG-19 into distilled water. The pH value of the dye-containing solution was adjusted with 0.1 M NaOH and 0.1 M HCl solutions and measured using a pH meter (pHS-3C, Leici Ltd, China). All the reagents used were of analytical grade.
Figure 1

Chemical structure of RG-19.

Figure 1

Chemical structure of RG-19.

Adsorption experiments

The batch RG-19 adsorption experiments were carried out in 100 mL stoppered conical flasks. The flasks filled with 0.1 g of Pani-Bent and 20 mL of RG-19 solution were shaken at a speed of 130 rpm over a period of time at a certain temperature in a shaker. At the end of the adsorption experiments, the solutions were centrifuged at 8,000 rpm for 10 min. The RG-19 concentration in supernatant was measured using an ultraviolet-visible spectrophotometer at the wavelength of maximum absorbance of 630 nm (UV-5100, Yuanxi Instruments, Shanghai, China) (Zuorro et al. 2013; Petrucci et al. 2015). The RG-19 adsorption by Pani-Bent was calculated by the following Equation (1): 
formula
1
where q is the mass of RG-19 adsorbed per unit Pani-Bent (mg/g); C0 and Ce (mg/L) are the initial and final RG-19 concentration, respectively; V (L) is the volume of RG-19 solution; and m is the weight of Pani-Bent (g).

Statistical analysis

The statistical analysis of the experimental data was conducted by means of a t test using SPSS (Version 17.0, IBM). The confidence level for statistical significance was 95%.

RESULTS AND DISCUSSION

Characterization of adsorbent

The FTIR spectra of Puri-Bent and Pani-Bent were recorded in the range of 4,000–400 cm−1 to prove the existence of surface functional groups. As is shown in Figure 2, Puri-Bent and Pani-Bent have a lot of the same groups. For example, absorbed water contributes to O-H stretching at around 3,700, 3,620 and 3,420 cm−1. The band at 1,033 cm−1 is due to the Si-O stretching (Saikia & Parthasarathy 2010). The band at 912 cm−1 is attributed to Si-O stretching of the silanol group. The bands at 792, 752 and 696 cm−1 are due to the mixed Si-O deformations and octahedral sheet vibrations (Quintelas et al. 2011). The bands at 536 and 469 cm−1 are ascribed to the Si-O-Al stretching and Si-O-Si bending (Saikia & Parthasarathy 2010). Compared with the Puri-Bent spectrum, two new peaks at 2,930 cm−1 and 2,850 cm−1, which are assigned to the C-H stretching of alkanes, appear in the Pani-Bent spectrum. The bands at 1,493 and 1,636 cm−1 are attributed to ring stretching vibration of benzenoid and C = C stretching in aromatic nuclei (Sanches et al. 2011; Vivekanandan et al. 2011), indicating the successful modification of the raw bentonite.
Figure 2

FTIR spectra of the Puri-Bent and Pani-Bent.

Figure 2

FTIR spectra of the Puri-Bent and Pani-Bent.

Figure 3 shows the XRD patterns of Puri-Bent and Pani-Bent. The d001 reflection at 2θ for Puri-Bent is 5.98 °, and the corresponding interlayer spacing is 14.77 Å. The 2θ of d001 reflection shifts to 5.66° for the Pani-Bent, and its interlayer spacing increases to 15.61 Å. Hence, XRD analyses results indicate that pani molecules did not enter adequately into the layers of bentonite structures.
Figure 3

XRD patterns of the Puri-Bent and Pani-Bent.

Figure 3

XRD patterns of the Puri-Bent and Pani-Bent.

Effect of pH

Solution pH can affect both the surface charges of the adsorbent and existing forms of dye molecules, which in turn affects the adsorption capacity (Wu et al. 2007; Zhang et al. 2014). Hence, the effect of pH on RG-19 uptake was investigated. The pH range set for the experiments was 5–9. Theoretically, the interactions caused by electrostatic forces between the negatively charged RG-19 molecules and positively charged active sites on the Puri-Bent surface are the main mechanism for the dye adsorption process, and the adsorption would be weakened with the increasing pH due to the fact that the negative charge of RG-19 and positive charge on the Puri-Bent surface decrease with rising pH. Also, the competition for the active sites between the dye molecule and OH in solution causes an adsorption decrease as well. Figure 4 shows RG-19 adsorption decreases from 42.8 to 40.6 mg g−1 as the solution pH increases from 5 to 9. Though the overall trend of RG-19 adsorption decreases with the rising pH from 5 to 9, the statistical result shows there is no significant difference in RG-19 uptakes between two adjacent pH values (p>0.05), indicating that a slight pH increase leads to a mild decrease in RG-19 adsorption by Pani-Bent in the pH range of 5–9. The same trend was observed when the bentonite modified with hexadecyltrimethylammonium was used to treat reactive black 5 (Jovic-Jovicic et al. 2013).
Figure 4

Effect of pH on RG-19 adsorption by Pani-Bent (C0 = 250 mg/L, T = 25°C, time = 24 h, pH = 5.9 ± 0.1).

Figure 4

Effect of pH on RG-19 adsorption by Pani-Bent (C0 = 250 mg/L, T = 25°C, time = 24 h, pH = 5.9 ± 0.1).

Effect of inorganic salts

During the dyeing process using reactive dyes, inorganic salts are often added to improve dye-fiber bonding by driving the dye molecules out of solution onto the fiber. Therefore, two inorganic salts, NaCl and Na2SO4, which are widely used in the dyeing process, were adopted to test their effects on RG-19 adsorption (Figure 5). The RG-19 uptake increases from 42.2 mg g−1 without salt to 45.3 mg g−1, 46.8 mg g−1 and 48.2 mg g−1 by addition of NaCl at doses of 4 g L−1, 8 g L−1 and 12 g L−1, respectively. When Na2SO4 is used at the same dose, the corresponding RG-19 adsorption is 42.7, 44.5 and 45.8 mg g−1. The results suggest that both NaCl and Na2SO4 can improve the RG-19 adsorption at all the three dosages, and the dye adsorption increases significantly with a rising dosage (p < 0.05). This result is similar to the removal of reactive black 5 by carbon F400, two bamboo based active carbons and peat reported by Ip et al. (2009); however, it is opposite to the adsorption removal of three reactive dyes by metal hydroxide sludge reported by Netpradit et al. (2004).The reason why the two salts can promote dye uptake is that the increased intermolecular forces, including van der Waals forces, ion-dipole forces and dipole-dipole forces, induced by the addition of the two inorganic salts prompt the aggregation of dye molecules, which in turn leads to the increase in the extent of RG-19 adsorption on the Pani-Bent surface (Al-Degs et al. 2008).
Figure 5

Effect of inorganic salts on RG-19 adsorption by Pani-Bent (C0 = 250 mg/L, T = 25°C, time = 24 h, pH = 5.9 ± 0.1).

Figure 5

Effect of inorganic salts on RG-19 adsorption by Pani-Bent (C0 = 250 mg/L, T = 25°C, time = 24 h, pH = 5.9 ± 0.1).

Adsorption kinetics

The effect of reaction time on RG-19 removal is shown in Figure 6. The fast adsorption of RG-19 by Pani-Bent takes place within the first 15 min. This phenomenon may be due to the fact that there are a large numbers of vacant active sites on the surface of Pani-Bent in the initial stage. The adsorption speed increases slowly until practically at equilibrium in 1,440 min, because fewer sites are available in the slow stage and the repulsive forces between the molecules absorbed on the surface of Pani-Bent and the free dye molecules in solution become stronger (Zhang et al. 2014).
Figure 6

Effect of reaction time on RG-19 adsorption by Pani-Bent (T = 25°C, pH = 5.9 ± 0.1).

Figure 6

Effect of reaction time on RG-19 adsorption by Pani-Bent (T = 25°C, pH = 5.9 ± 0.1).

In order to investigate the adsorption processes of RG-19 by Pani-Bent, adsorption kinetic models including pseudo-first-order, pseudo-second-order and Elovich diffusion equations were adopted (Liu & Zhang 2015). The respective linear forms of the three equations are expressed as Equations (2)–(4): 
formula
2
 
formula
3
 
formula
4
where qt and qe (mg g−1) are the amount of RG-19 adsorbed at time t and at equilibrium; k1 (min−1) and k2 (g mg−1 min−1) are the equilibrium rate constants of the pseudo-first-order equation and pseudo-second-order equation, respectively; α (mg g−1 min−1) is the initial adsorption rate; and β (g mg−1) is the desorption constant. The two rate constants can be calculated from the plot of experimental data.

The kinetic parameters obtained from the three equations are shown in Table 1. The value of the correlation coefficient R2 of the pseudo-second-order model is highest among the three coefficients. The qe,cal (41.32 mg g−1 for 250 mg L−1, 52.91 mg g−1 for 500 mg L−1) calculated from the equations agree very well with qe, exp (42.24 mg g−1 for 250 mg L−1, 55.23 mg g−1 for 500 mg L−1) obtained from the experiments. As a result, the pseudo-second-order model is best suited to describe the adsorption process of RG-19 by Pani-Bent. The results suggest that the chemical adsorption or chemisorption might be the rate determining step, and the valency forces through sharing or exchange of electrons between the dye molecules and the active sites of the Pani-Bent adsorbate might be involved in the reaction (Örnek et al. 2007).

Table 1

Parameters of the three kinetic equations for RG-19 adsorption by Pani-Bent

Kinetics equations Parameters 250 mg L−1 500 mg L−1 
Pseudo-first-order qe 16.40 23.09 
k1 0.005 0.004 
R2 0.820 0.805 
Pseudo-second-order qe 41.32 52.91 
k2 0.002 0.001 
R2 0.997 0.999 
Elovich α 1870.7 1039.1 
β 0.313 0.227 
R2 0.977 0.992 
Kinetics equations Parameters 250 mg L−1 500 mg L−1 
Pseudo-first-order qe 16.40 23.09 
k1 0.005 0.004 
R2 0.820 0.805 
Pseudo-second-order qe 41.32 52.91 
k2 0.002 0.001 
R2 0.997 0.999 
Elovich α 1870.7 1039.1 
β 0.313 0.227 
R2 0.977 0.992 

Adsorption isotherm studies

The adsorption isotherm gives an idea of the distribution of the adsorbate molecules between the liquid-solid interface when the adsorption reaches its equilibrium, and adsorption parameters can be also obtained by analysis of isotherm data (Peng et al. 2005; Hameed et al. 2008). So, two widely used isotherm equations, the Langmuir and Freundlich models, were used to fit the data obtained from the experiments. The linear forms of the two models are expressed as Equations (5) and (6): 
formula
5
 
formula
6
where (mg g−1) is the amount of RG-19 adsorbed per unit mass of Pani-Bent; (mg L−1) is the RG-19 concentration at equilibrium; (mg g−1) is the maximum adsorption capacity; b (L mg−1) is the Langmuir constant related to binding energy; and and n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively.
The adsorption isotherms are presented in Figure 7, and the calculated constants of the two models are shown in Table 2. The correlation coefficients R2 of the Langmuir model are higher than those of the Freundlich model, indicating the Langmuir model fits the data better than the Freundlich model. RL, a dimensionless constant separation factor was used to further understand the characteristic of the adsorption process. The factor is defined as Equation (7): 
formula
7
where C0 (mg L−1) is the initial RG-19 concentration; and b (L mg−1) is the Langmuir adsorption equilibrium constant.
Table 2

Adsorption isotherm constants for adsorption of RG-19 by Pani-Bent

T(K) Langmuir
 
Freundlich
 
R2 qm b R2 n kf 
278 0.999 49.02 0.162 0.919 5.102 3.558 
291 0.997 53.48 0.135 0.959 4.717 3.567 
318 0.995 58.14 0.118 0.956 4.241 3.527 
T(K) Langmuir
 
Freundlich
 
R2 qm b R2 n kf 
278 0.999 49.02 0.162 0.919 5.102 3.558 
291 0.997 53.48 0.135 0.959 4.717 3.567 
318 0.995 58.14 0.118 0.956 4.241 3.527 
Figure 7

Adsorption isotherms of RG-19 by Pani-Bent at three temperatures (time = 24 h, pH = 5.3 ± 0.2).

Figure 7

Adsorption isotherms of RG-19 by Pani-Bent at three temperatures (time = 24 h, pH = 5.3 ± 0.2).

The values of RL calculated for different initial concentrations at different temperatures are given in Table 3. The fact that 0 < RL < 1 for all of the experiments indicates the favorable nature of the adsorption of RG-19 by Pani-Bent.

Table 3

RL values based on the Langmuir model

T(K) RL
 
100 mg L−1 140 mg L−1 180 mg L−1 220 mg L−1 260 mg L−1 300 mg L−1 340 mg L−1 380 mg L−1 420 mg L−1 
278 0.058 0.040 0.033 0.027 0.023 0.020 0.017 0.016 0.014 
291 0.993 0.050 0.040 0.033 0.028 0.024 0.021 0.019 0.017 
318 0.895 0.960 0.960 0.970 0.974 0.977 0.980 0.982 0.983 
T(K) RL
 
100 mg L−1 140 mg L−1 180 mg L−1 220 mg L−1 260 mg L−1 300 mg L−1 340 mg L−1 380 mg L−1 420 mg L−1 
278 0.058 0.040 0.033 0.027 0.023 0.020 0.017 0.016 0.014 
291 0.993 0.050 0.040 0.033 0.028 0.024 0.021 0.019 0.017 
318 0.895 0.960 0.960 0.970 0.974 0.977 0.980 0.982 0.983 

Thermodynamics

Thermodynamic parameters of the adsorption were calculated using Equations (8) and (9) (Tütem et al. 1998; Huang et al. 2007; Shao et al. 2012). 
formula
8
 
formula
9
where qe (mg g−1) and Ce (mg L−1) are the amount of RG-19 adsorbed per unit mass of Pani-Bent, and RG-19 concentration at equilibrium, respectively. ΔH° (kJ mol−1), ΔS° (kJ K−1 mol−1) and ΔG° (kJ mol−1) are the enthalpy, entropy and the standard Gibbs free energies, respectively. R (8.314 J mol−1 K−1) is the ideal gas constant, and T (K) is the temperature in Kelvin.
The values of Ce are calculated using the method of Tütem et al. (1998). ΔH° and ΔS° are obtained from the slopes and intercepts of the plots of lnCe vs. 1/T (Figure 8). The values of ΔG° are calculated from Equation (8). The thermodynamic parameters including ΔH°, ΔS° and ΔG° are listed in Table 4. The negative values of ΔG° indicate the feasibility of the process and the spontaneous nature of the adsorption. The decreasing ΔG° with the rising temperature indicates better adsorption at higher temperature (Chen & Wang 2006). The positive values of ΔH° indicate the adsorption is endothermic.
Table 4

Thermodynamic parameters for RG-19 adsorption by Pani-Bent

Qe (mg g−1ΔH° (kJ mol−1ΔS° (J K−1 mol−1ΔG° (kJ mol−1)
 
278 K 291 K 318 K 
28 0.887 0.013 −2.825 −2.999 −3.360 
30 1.438 0.015 −2.601 −2.790 −3.182 
32 2.094 0.016 −2.353 −2.561 −2.993 
Qe (mg g−1ΔH° (kJ mol−1ΔS° (J K−1 mol−1ΔG° (kJ mol−1)
 
278 K 291 K 318 K 
28 0.887 0.013 −2.825 −2.999 −3.360 
30 1.438 0.015 −2.601 −2.790 −3.182 
32 2.094 0.016 −2.353 −2.561 −2.993 
Figure 8

Plots of lnCe versus 1/T for adsorption of RG-19 by Pani-Bent.

Figure 8

Plots of lnCe versus 1/T for adsorption of RG-19 by Pani-Bent.

CONCLUSIONS

The Pani-Bent was synthesized, characterized and used for RG-19 removal in this study. The RG-19 uptake decreased slightly with increasing solution pH. Both NaCl and Na2SO4 increased the dye adsorption with rising salt dosage, resulting from aggregation of the dye molecules in solution. The kinetic data fitted the pseudo-second-order kinetic model best among the three models, and isotherm data were described better by the Langmuir model than by the Freundlich model. Adsorption of RG-19 was found to be spontaneous and endothermic at the temperature range of 278–328 K in this study, as indicated from the negative values of ΔG° and ΔH°. The results indicated that the Pani-Bent is an effective adsorbent for anionic dye removal from water.

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